Limit scope of arguments to service_control() in service module

This fixes handling of arguments (module argument) in service_control().
It is now locally scoped such that modifications to it, in the case of a
systemd host, do not later impact subsequent calls to service_control().
See issue #2449.

Without patch:
ansible-service[6177]: Command /usr/bin/systemctl stop cups cups , daemonize True

With patch:
ansible-service[6475]: Command /usr/bin/systemctl start cups , daemonize True

I also updated the comments to reflect the case where systemd is really
being called.